Bit Index Explicit Replication (BIER) is a multicast solution that is proposed by Internet Engineering Task Force (IETF) draft titled, “Multicast using Bit Index Explicit Replication,” by I J. Wijnands, et al., published on Sep. 22, 2014 and, IETF draft titled, “Encapsulation for Bit Index Explicit Replication in MPLS Networks,” by I J. Wijnands, et al., published on Sep. 22, 2014, which are both hereby incorporated by reference as if reproduced in their entirety. A BIER domain is built on top of Interior Gateway Protocol (IGP). The BIER domain requires that BIER Forwarding Routers (BFRs) use bit string information from BIER packets in order to compute replication paths locally. BFRs are also required to modify the bit string on the packet prior to forwarding the packet to the next-hop BFR.
A bit string is created by a BIER Forwarding Ingress Router (BFIR) that is attached to a multicast source. A BIER packet header may be read using the bit string to capture an egress router's address information. BFRs use the bit string information to calculate a replication path, to modify the bit string, and to forward the packet to a next-hop BFR until a data packet is transmitted from the BFIR to the BIER Forwarding Egress Routers (BFERs). For example, the bit string can be modified to prevent looping and duplication by ensuring the bit string comprises only BFERs that the next-hop BFR needs to forward towards. BIER does not require an explicit tree-building protocol for multicasting and does not require intermediate nodes to maintain per-flow states.
When a multicast data packet enters a BIER domain, the ingress router (e.g., BFIR) determines the set of egress routers (e.g., BFERs), encapsulates the packet in a BIER header, and transmits the packet to its adjacent BFR. The adjacent BFR will determine the destination BFERs and the next-hop adjacency for each of the BFERs. For example, the adjacent BFR will look up a bit string, a multiprotocol label switching (MPLS) label, and underlay network information. The adjacent BFR will also make a copy of the packet, clear the bit string, modify the BIER header, and transmit the packet to the next-hops. Every packet transmitted over a BIER domain is inspected and modified on each BFR prior to forwarding the packet to the next-hop. The repeated inspection and modifications on each BFR are time consuming, waste resources, reduce multicast transport speeds, and degrade network performance.